The present invention relates to electric watthour meters and, more particularly, to terminals for connecting watthour meters to external circuits.
Conventional watthour meters as used in the United States generally fall into two categories: detachable and bottom-connected. The present invention is particularly directed toward detachable meters.
According to governing standards of the American National Standards Institute (ANSI), a detachable (socket-mounted) watthour meter includes a generally circular base supporting the metering and registering components. Bayonet-type blade terminals pass through the back of the base, arranged for insertion into mating jaws of a meter socket, or other detachable meter mounting device. The ANSI standard also governs the locations and dimensions of the portions of the terminal blades external to the base. The shape of the portions of the terminals internal to the watthour meter are at the discretion of the manufacturer.
A conventional electro-mechanical watthour meter employs a conductive metal disk driven as the rotor of a small induction motor by the interaction of magnetic fluxes generated by opposed voltage and current stators. The voltage and current stators are conventionally disposed facing and spaced apart from each other with the metallic disk in the gap between them.
The voltage and current stators, as well as the elements supporting the disk, are mounted on a frame generally made of cast aluminum. The frame is, in turn, attached to the base.
One, two or three sets of voltage and current stators are found in conventional watthour meters. The voltage and current stators, as well as other elements within the watthour meter, occupy a substantial part of the limited space within the watthour meter. The locations, shapes and sizes of the internal elements are governed by standards and by technical requirements and are generally beyond the control of the designer.
Each current stator consists of an assembly of thin iron laminations forming a U-shaped magnetic core, with two magnetic poles facing the disk. A current coil consisting of a few turns of large-diameter insulated conductor is wound about one leg of the U-shaped core to magnetize the core and produce a magnetic flux at its poles in proportion to the load current. A terminal is connected to the end of each of the two leads of a current coil for making external connections to the meter socket.
The current coil and its leads carry the entire load current to be measured and are thus given large cross sections to avoid excessive heating. Such large cross sections make the leads rigid and incapable of substantial manual bending or manipulation to fit them to the internal parts of a meter. It thus becomes a problem to route the leads of the current stator around internal elements, over which the designer has little if any control, for connection to an internal part of the terminal.
Co-pending U.S. patent application Ser. No. 711,716 discloses an improved current coil in which the center line of the current coil coincides with the center line of the terminal blade, thereby permitting a single coil design to serve for current stators in both left and right positions in the watthour meter. This places even further restrictions on the routing of the leads of the current stators.
A coil of each voltage stator is also connected to terminals. However, the voltage-stator coil is made up of many turns of a fine-gauge wire, thereby attaining a high impedance. Thus, only a small current flows in the voltage coil, and the problem of attaining adequate electrical connection between the voltage coil and the terminals is simplified. In many cases, satisfactory connection can be made with flexible insulated wire attached to the terminals with screws or soldering.
The routing requirements for the current-coil leads, combined with their stiffness, conventionally dictate that the ends of the leads lie at positions offset from a plane defined by the respective blades of the terminals. It thus is conventional to form terminals with internal offsets for interfacing with their current-coil leads while maintaining the external configuration according to ANSI standards.
Different watthour meter designs have sufficiently different lead-routing constraints that the internal terminal offsets vary from watthour meter to watthour meter. It is thus necessary for a manufacturer to employ a number of different terminal designs for meeting the offset requirements of its line of watthour meters. Further variations in terminal design are often required to accommodate current-coil conductors of different ampere ratings. Still further variations may be required to permit screw or solder attachment of voltage-coil leads to the terminals.
The resulting variety of terminals represents a significant cost in design, tooling, machine set-up time, inventory and administration. The short manufacturing runs engendered by such variety leads to manufacturing inefficiency.